(57) In a method for rotating image data of a source rectangle area of a document by a specified rotation angle and storing said rotated image data in a destination rectangle area of a storage means, said storage means being partitioned by boundaries separated from each other by K bits, comprising steps of:

detecting a space between an edge of said destination rectangle area and said boundary in said storage means;

storing said image data of said source rectangle area corresponding to an area of said edge into a line buffer which is partitioned into plural blocks each of which has a size of K×K bits by adding a space which corresponds to said space in said storage means;

storing said K×K bits of one of said blocks of said line buffer into a rotation buffer of K×K bits; and

fetching each of the K bit groups including said image data from said rotation buffer in a direction in accordance with said rotation angle and storing said K bit groups in said destination rectangle area of said storage means.